Why Would Aliens Want the Earth?

Basic Assumptions

If an incoming alien race has faster than light travel, teleportation,
reactionless drives, direct conversion of matter to energy, anti-gravity, or
other staple science fiction technologies, all bets are off. We have no idea
what they'd be able to do because we have no idea how those technologies could
even be possible, given our understanding of the laws of physics. And if you're
about to pooh-pooh that as impossibly myopic, why not first prove you know
anything about the laws of physics? Say, explain how Trojan orbits work, in
detail, as in, do the math. Or explain how Maxwell's Equations lead to radio.

We can assume that any race capable of crossing interstellar space will be
able to do anything we can do or even remotely contemplate doing. On the other
hand, space travel won't be trivial, even for them. Traveling between ships in
their fleet will probably be easy out there in zero gravity, but getting to
speeds capable of crossing space in a reasonable time, and slowing down once
they reach their destination, will take serious effort. Any civilization capable of crossing interstellar space will probably be able
to travel freely between planets, but it will still take a lot of energy and
time. Landing on massive planets, and getting off again, will also be a
significant effort. Terraforming planets will likewise take a lot of effort and,
even more important, time.

If Earth-like planets are extremely rare and we just happen to be on one that
the aliens need, well, sucks to be us. On the other hand, if earth-like planets
are at all common, the odds are very much against one of them having an
intelligent civilization. If humans last 5 million years, that will be less than
1/1000 of the age of the earth. The odds of aliens coming here during our window
of existence are about 1/1000.

So we'd have to assume that any
extra-solar visitors come to our solar system for a specific reason, which might
be to colonize, gather resources, or explore. (Or possibly, as in Arthur C.
Clarke's Rama novels, simply to use the Sun for a trajectory change.)
We should pretty much assume that they are ahead of us technologically in just
about everything. Anything we have on the drawing boards, they can do.

They certainly will have technology for observing planets around other
stars, since we ourselves have begun to do it. If they have telescopes millions
of kilometers apart flying in formation with their ships, they can combine the
images to obtain the effective resolution of a telescope millions of kilometers
in diameter. We already have conceptual ideas for doing this. So quite likely
they would have pretty good images of all the planets in our solar system.

If they pick up our radio and television signals, they'd probably know that
there is at least that level of technology in our solar system. Most likely
they'd pick up only the carrier wave, with the actual signal information too
weak to pick out. Then again, in zero gravity, there would be nothing to stop
them from building a radio dish kilometers across. We have to assume they are at
least as good at cryptography as we are, so if they could pick up the
information, they would likely be able to decipher it.

Resources

In the unlikely event they simply come here blind because they just picked a
G-class star at random and didn't do any reconnaissance on the way in, it's very
likely that they wouldn't lightly pack up and go to another star, since
interstellar travel will be resource and time intensive. And, of course, if they
know what's here, then they came here for a specific reason. But if their
primary goal is resources, and they can travel easily between planets, there are
much better sources than the Earth.

Water: Icy moons are every bit as convenient as Earth
as sources of water. Comets are probably too small and widely scattered to
be of much use, although a single comet can supply many cubic kilometers of
water.

Metals: Nickel-iron asteroids can supply far more metal
than our iron mines, with the added advantage that the smelting is mostly
already done. Rarer metals can be extracted during the refining of nickel
and iron, just as we do on Earth. Asteroids and comets have the added
advantage of negligible gravity.

Oxygen: Obtain it by electrolyzing water, or as a
by-product of smelting silicates.

Hydrocarbons: Assuming they use hydrocarbons for some
energy needs or a base for creating other organic chemicals, why drill on
Earth when there's a whole planet of it at Jupiter, and lakes of
hydrocarbons on Titan?

Energy: Pave Mercury with solar panels, or simply build
vast arrays in space. Use the
harvested energy to fabricate still more panels, as well as whatever else
you need.

If they come to our solar system to gather extraterrestrial resources, earth
might be irrelevant to them, just like us going into the woods to pick berries is
irrelevant to some bird nesting in a nearby tree. We are very unlikely to be a threat, or even a nuisance. Aww, isn't that
cute? We have itty-bitty nucwear weapons. They can travel between planets and we
can't go beyond earth orbit. They could strip-mine Mars and we'd be able to
do little more than watch. We might launch robotic missions to watch them, and
maybe pick up some useful ideas, assuming the aliens allow them to get close.

We are also unlikely to have anything cultural or technological that they
need. Iron and steel? With nickel-iron asteroids for the taking? Electronics?
They probably maxed out Moore's Law ages ago. Some of our cultural artifacts
and art works may be of interest.

So Who Needs Earth?

There are a few reasons why extraterrestrial visitors might come specifically to Earth
apart from curiosity or exchanging greetings:

Hydrothermal Ores: The
problem with using the Moon or asteroids as sources of minerals is that a
lot of our ores on earth involve water in some way. Copper, zinc and lead
are mostly deposited by hot solutions. Aluminum is concentrated by
weathering of aluminum rich rocks in tropical environments. Those deposits
might make terrestrial sources more attractive than extraterrestrial ones.

Lithophile elements: Plate tectonics and geochemical
recycling have made the earth's interior more homogeneous and has concentrated
certain elements in the crust. These tend to be atoms with large ionic
radii, high ionic charges, or both. Things like lithium, beryllium, boron,
uranium, thorium, cesium, and rare earths might be easier to extract from
Earth rocks than asteroids. We now know that planets abound, but watery
earth-like planets may not be.

Organic chemicals: If you want complex organic
chemicals, it might be easier to harvest them from the biosphere rather than
synthesize them from scratch. Maybe we have things they didn't know about.
Most interesting this - what do you call it? - cocaine.

Convenient Working Environment: If you're an oxygen
breather with human-like environmental needs, it may be a heck of a lot easier to
land on Earth and do things than terraform Venus or build gigantic domes on
the Moon.

R & R: No matter how big your starship is, even if it's
got holodecks, it has to get boring after a while. If they can survive on
Earth, they'll very likely want to come down for some breathing room and to
see new scenery.

A Place to Live: Bad news. There have to be lots of
Earth-like planets without intelligent life. After all, Earth was like that
for over 99.9% of its history. So why pick Earth? Assuming they didn't come
here just because they enjoy destroying other intelligent life forms,
something probably went very wrong. Either we're the nearest Earthlike
planet and they need it very badly, or they didn't know we were here, or
we're so far below their level we don't rate as intelligent. If their life support is failing and there are twenty billion
of them, we're toast. But if there aren't too many, and they're willing to
work with us, we could probably find some open spaces for them while they
terraform Mars or Venus.

Encounter Scenarios

They Come and Go

They come into the solar system, harvest a few comets and metal rich
asteroids, suck hydrocarbons out of the Gas Giants, unfurl giant solar arrays to
replenish their energy storage, and leave without contacting us at all. If this
had happened even in 1900, we probably would have not even noticed it, but today
we would. We'd certainly know we weren't alone, but on the other hand we'd be
more alone than ever, since there are races that travel among the stars and
don't even consider us worth contacting.

We are Superior

Forget this. They came here. We can't go there. They will likely be ahead of
us in everything. the only exception might be a District 9 scenario,
where a small alien contingent arrives, either as refugees or because they had a
catastrophic failure of their ship.

We're OK, They're OK (Star Trek)

The best of all possible universes. But we'll still be grossly inferior to
them in technology. But they'll treat us nice, and maybe let us share their
stuff. That may not be a good thing (see Culture Shock, below).

War of the Worlds (or V, or the Borg)

We're food, raw material, or an impediment. Maybe we can make ourselves
enough of a nuisance that they leave us alone if we stay out of their way. On
the other hand, with our industrial base gone, we're back to rocks and sharp
sticks pretty soon, and what kind of a fight can we put up then?

We might make a good ad hoc food source, especially as a way of exterminating
us, but we'd be lousy as a continuing
source of food. We escape, attack our handlers, and grow slowly. As opposed to
cows, who are dumb, docile, fast growing and capable of eating grass.

Watership Down

Watership Down was a 1972 novel by Richard Adams about a colony of rabbits
that had to find a new home. It begins with a vision of bulldozers tearing up
the field the rabbits lived in. The humans didn't hate the rabbits, or even care
about them. They were just in the way.
I consider this the most likely scenario. They come to Earth
because they want copper and aluminum, and they need open space to set up
facilities to fabricate things for their ships, and why build a huge enclosed
dome on the Moon when Earth already has a comfortable working environment? So
they see our copper mines and strip them out. They detect other hydrothermal
ores forming underground and dig them out, as well. And they don't care that
it's Yellowstone National Park. Maybe they'll realize we already have a lot of
refined copper and root it out of our cities, just like the Spanish plundered
New World silver. They don't hate us, and they may even try to avoid gratuitous
harm, but in comparison to them, we're about as smart as a Labrador retriever,
and we just don't rate on their scale. Some of them might study us as exercises
in exobiology, but their children can write things more advanced than Hamlet
and make up songs more elaborate than anything by Beethoven. Our art will
interest them less than the cheapest tourist trinket impresses us. They won't go
out of their way to hurt us, because advanced beings don't unnecessarily
mistreat lower life forms (although kids will step on bugs), but if they want something, they'll take it. If they
want to clear Florida to build a solar power array, or use Iowa to grow some
crop important to them, they'll do it. We'll just have to move, just like the
bunny rabbits did.

Culture Shock

Maybe benevolent aliens won't be such a good thing after all. Even when
advanced civilizations have encountered primitive ones on earth peacefully, the
results have generally been rough on the primitive civilization.

For openers, what's the point of research when the aliens already have the
answer to any question we might ever ask? We could easily become passive users
of their knowledge.

Then there's the impact on our beliefs. If they have indisputably been around
for millions of years, that pretty much demolishes a recent creation, right? Or
maybe not. It's very likely we'd see the rise of powerful movements denouncing
the aliens as liars or hoaxers and demanding an end to contact with them. What
if they've had a peaceful, benign, just civilization for hundreds of thousands
of years, superior to us in every way, and have never, ever had the concept of a
god? What if they have an extremely dogmatic religion, especially one that
meshes with some of our more dogmatic creeds? What if they have rigid sex roles,
or rigid social classes, or practice slavery? What if they have no
limits on sexual behavior whatsoever?

Finally, some humans will certainly interact more closely with the aliens
than others. If the aliens aren't careful, they can create such an economic
disparity that humanity ends up becoming a wholly owned subsidiary of Aliens,
Inc. Even on earth, anthropologists have noted there are clearly defined zones
of influence around outposts of advanced civilizations. Some humans would be
almost wholly assimilated. Others would find that proximity to alien knowledge
gives them powerful advantages over less favored humans, either in trade or in
the use of force.

That assumes that the aliens are benign or at least neutral. If they're here to exploit us or at least get
what they want without regard for us, then things will get a whole lot nastier. Because there will certainly
be humans who will work for whatever rewards the aliens offer. They'll
strip-mine Chicago for the metals, in exchange for power or sex. Or they'll act
as overseers to keep the rest of us in line.